The invention relates to a colour display device provided with a colour display tube having a display window, an electron gun and a deflection unit, which electron gun comprises cathodes, a beam-forming section, a focusing electrode and a final electrode, viewed in the direction from the electron gun to the display window, to which voltages are applied during operation, said electron gun generating an electron beam, during operation, that is deflected by the deflection unit to scan the display window in lines so as to form a picture, the colour display device further comprising electronic means generating a video signal at a pixel frequency.
A colour display device as described in the opening paragraph can for instance be provided with a colour display tube as disclosed in U.S. Pat. No. 5,818,157. The electron gun according to this prior-art specification comprises cathodes, a beamforming section having a plurality of electrodes for extracting the electrons from the cathodes and for forming the electron beams, which enter the main lens that is formed by the focusing electrode and the final electrode.
Such a colour display device drives the colour display tube with varying voltages on the cathodes and static voltages on the other electrodes. The varying voltages on the cathodes determine the beam currents, which have a more or less linear relationship with the light output of the colour display device.
In practice, these colour display devices have some limitations. For instance, it appears that when the light output changes also the sharpness of the picture changes. This is an unwanted effect and it deteriorates the focus performance of the colour display tube.
It is an object of the invention to provide a colour display device of the kind described in the opening paragraph, which is capable of creating a picture of improved harpness by overcoming the mentioned limitation of the prior-art colour display device.
This object is realized with a colour display device of the invention, that is characterized in that the voltage on the focusing electrode is varied as a function of the voltages on the cathode.
The invention is based on the insight that the voltage needed for focusing the electron beam is dependent on the beam current. This beam current is determined by the voltage applied to drive the cathode, that is the cathode voltage. This makes it advantageous to have the cathode voltage determine the voltage on the focusing electrode, that is the focus voltage. In this way, it is achieved that the electron beam is well focused for all beam currents.
In a preferred embodiment, the voltage on the focusing electrode varies at the same rate as the pixel frequency. In this embodiment, the focus voltage is adjusted to the cathode voltage for every position on the display window, that is for each pixel. This results in a picture that is in focus for all light output levels at all positions on the display window. For this embodiment, it is necessary to adjust the focusing voltage at the same rate as the pixel frequency. The pixel frequency, or video frequency, is the frequency needed for driving the individual pixels of a colour display tube. This pixel frequency is proportional to the product of the number of pixels and the frame frequency. The frame frequency gives the number of times the picture is refreshed per second. The pixel frequency may be quite high, for instance, in high-resolution computer monitors higher than 100 MHz.
In a further embodiment, the voltage on the focusing electrode during scanning a line of a picture is a function of the average of the voltages on the cathodes during scanning said line.
On the one hand, this embodiment renders less accurate results compared to the preferred embodiment, because for a given line the voltage on the focusing electrode is. fixed. On the other hand, it needs a much lower frequency for adjusting the voltage on the focusing electrode. The cathode voltage across an entire line is measured, the average cathode voltage is calculated and this value is used for determining the adjustment of the voltage on the focusing electrode. This procedure requires an electronic memory, because the data of the cathode voltage on a line have to be collected to determine the accompanying voltage on the focusing electrode, and this has to be done before the information of this line is displayed.
In a still further embodiment, the voltage on the focusing electrode during scanning the lines of a picture is a function of the average of the voltages on the cathodes during scanning the lines of a picture. In this case, the cathode voltage is also averaged over the lines, so that an average cathode voltage over an entire picture, or as it is often called, a frame, is obtained. This embodiment is even less accurate, because now the focusing voltage is fixed for an entire picture. The frequency with which the focusing voltage is adjusted is low, namely the frame frequency. Although the focusing voltage is constant throughout a picture, this embodiment still is a significant improvement on the prior art, where the focusing voltage is static with respect to time.
In another embodiment, the colour display device comprises an electronic memory containing data describing the relation between the voltage on the cathodes and the voltage on the focusing electrode.
For adjusting the focusing voltage as a function of the cathode voltage, it is required to know the relationship between the cathode voltage, the beam current and the focusing voltage. This relationship is programmed, for instance in the form of a table, in an electronic memory, so that, at a certain cathode voltage, the corresponding focusing voltage is read from the memory.